Weapons were not stored inside these facilities. These are "A Structures",
used for storing nuclear capsules inside special carriers known as "birdcages."
These carriers were pressurized to keep contaminants out, and were fitted
with standoff struts to prevent any criticality incidents with other capsules.
More pics and info (most informative):

Significance: Building 925 represents an important link in the design and
development of nuclear weapons storage facilities during the early Cold War
period. It is one of only five alert nuclear weapons storage facilities of
this type built in the United States. Building 925 is a distinguished
contributing element of the Armed Forces Special Weapons Project Q Area at
Travis Air Force Base, which has been recommended as a historic district
eligible for inclusion in the National Register of Historic Places under
criterion c.

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Three of the alert weapons storage facilities remain at Barksdale Air Force
Base, LA (below).